Electrophoresis involves the separation of charged molecular species in an electrical field. An electric field is applied to a macromolecular mixture, causing the mixture to be separated into fractions. Gel electrophoresis involves the migration of charged macromolecules through a porous gel under an applied electric field. Electrophoresis occurs because of differences in mobility of the different molecular species in the gel when subjected to electromotive force. Usually separation is based on differences in molecular weight, but can be based on differences in some other parameter, such as electrical charge.
Electrophoresis is a particularly desirable tool for separation or fractionation of macromolecular mixtures in which the molecular weight of the species present are from about 10,000 to about 1 million atomic mass units. Gel electrophoresis is a particularly suitable technique for fractionating protein mixtures, including DNA and RNA mixtures used in genetic studies. While electrophoresis has some limited use as a preparative tool, it is more widely used as an analytical tool.
Gel electrophoresis apparatus may be classified into two general types, vertical and horizontal. Both types have been extensively described in literature and patents. Vertical electrophoresis devices have an advantage in that cleaner separations and smoother flow of the macromolecular mixture to be fractionated are generally easier to attain in vertical apparatus than in horizontal apparatus due to greater electrical continuity between the electrodes and the gel.
U.S. Pat. No. 4,707,233 shows a representative vertical gel electrophoresis device. The apparatus shown therein comprises an open top container or tank having a basket-like gel cassette holder which is placed within the container or tank, a vertical gel cassette extending through the bottom wall of the gel cassette holder, a gasket forming a virtually fluid-tight joint between the gel cassette and the gel cassette holder, and oppositely charged electrodes on opposite sides of the non-conductive gel cassette holder walls so that electric current must pass through the gel matrix. The gel cassette is comprised of a pair of parallel plates joined together along their vertical edges by spacers, so that the gel cassette is open at the top and bottom but not along the sides, and a sample holder resembling a comb or a series of hollow tubes for directing macromolecular mixture through the gel matrix and thus between the walls of the cassette. The gel cassette may be held together by any suitable means, such as strips of vertically extending adhesive tape which overlay the outside surfaces of the spacer and the vertical sides of the plates, or the vertical plates may be fused together. The interior space of the gel cassette is filled with a suitable electrophoresis gel before the device is put into operation.
Various problems are associated with known gel cassettes. One problem involves the removal of the gel from the cassette for further processing after the electrophoresis run is completed. As conventional gel cassettes are often held together by strips of adhesive tape or by fusing the plates together, in order to remove the gel, the adhesive tape must be removed or the plates must be broken apart. The force required to disassemble the cassette by such methods can cause the plates to bend or otherwise interfere with the gel thereby disrupting or partially destroying the gel.
Another problem with the conventional gel cassettes, especially disposable gel cassettes, is that such cassettes are often comprised of a plastic material which may be very slippery. The slipperiness of the plastic material may result in part or all of the gel sliding out of the cassette during the electrophoresis run, thereby ruining the electrophoresis run.
An additional problem with conventional gel cassettes involves the combs used for directing macromolecular mixture through the gel cassette. In many conventional gel cassettes, the comb is inserted into the cassette prior to pouring the gel into the cassette. If a large number of electrophoresis runs are to be done wherein each run requires a different number of samples, cassettes having combs of differing numbers of sample wells must be kept on hand. Storing such large numbers of cassettes can be costly and cumbersome. Further, once the gel has been poured into the gel cassette, one is not able to change to a different comb having a differing number of sample wells.
Other prior art gel cassettes have attempted to overcome this problem by using combs which may be placed on top of the gel cassette after the gel is poured. The sample wells of the comb are in the shape of a shark tooth. The comb is placed at the top of the gel cassette in the opening between the two parallel plates, such that the pointed end of the shark tooth is placed into the gel. Samples of solutions to be fractionated are injected into the shark tooth shaped sample wells. The problem with these combs is that inserting the pointed end into the gel can damage the gel. Further, neither these gel cassettes nor the gel cassettes discussed above, wherein the comb is installed prior to the gel being poured, offer any protection to the top of the comb during the electrophoresis run. Dust or other undesirable particles may enter through the top of the sample wells and migrate into the gel disrupting or damaging the gel.
It is desirable to have a cassette for gel electrophoresis which may be easily disassembled without disrupting or damaging the gel and which prevents the gel from sliding out of the cassette. It is also desirable to have a cassette for gel electrophoresis which allows for the easy insertion of a comb into the cassette after the gel has been poured without damaging the gel and which protects the comb from having unwanted material enter into the comb and hence into the gel.